Ion chromatography

The ion exchange chromatography, often only ion chromatography is an analytical tool in chemistry and biology. Using this chromatographic method substances can be separated based on their charge. Are charged functional groups that have reversible counter ions (cations and anions when the cation exchanger anion exchanger) bound to a polymeric matrix.

A brief history

Ion exchange is one of the oldest separation processes described in the literature. Than the previous ion-exchange chromatography is considered the classic column chromatography, wherein the sample is more or less separated into individual components and is collected using an automatic fraction collector. Now the collected samples have often been examined by wet chemical methods. The Eluensvolumen often amounted to several liters. The enormous increase in performance is due to Small. He developed reproducible ion exchange resins with low capacity and high chromatographic efficiency. This led to a reduction in injection volumes, ten to 100 microlitres, so that the resolution could be increased, to obtain very small signals. Important improvements provide automatic detection, which now enables continuous recording of signals, and the introduction of conductivity detection dar.

The ion chromatography system

With the pump, the mobile phase is fed through the entire system. The inlet of the sample to be analyzed is performed by means of a Schleifeninjektors. The sample is first injected, and then transported to the opening of the valve by the mobile phase for the separation system. Injection volumes of between five and 100 microliters are typical. The most important component of the ion chromatographic system is the analytical column. The carrier materials are usually quartz glass (coated ), ethylene- tetrafluoroethylene ( ETFE), epoxy resins, divinylbenzene polymers, or polyetheretherketone (PEEK ) having a low capacity of functional groups. The ion exchange is usually operated at ambient temperature. The detector used for the qualitative and quantitative detection of an analyte. Since there is a relative method, a calibration for quantification has to be carried out. Most commonly, the conductivity detector is used, beside still UV/VIS-, amperometric and fluorescence detectors are used. The system can, however, also be coupled to a mass spectrometer via a ionizers. The object of the conductivity detector preceded connected suppressor is in the suppressing of the intrinsic conductivity of the eluent, so that the detection limit of the analyte is greatly improved. Through the suppressor of eluent is usually removed from the analyte stream and discarded. This is therefore only necessary for the use of a conductivity detector. The detectors often have nowadays a serial interface so that they can be connected to a PC.

The mobile phase, and the eluent

As a mobile phase, all liquid or dissolved substances, which are used as fuel mixture through the ion chromatography system using the pump, respectively. The mobile phase is usually made of analyte ( the ions to be analyzed are in the picture with "A" marked) and the eluent ( the Eluentionen are marked with " E"). The object of the eluent is to be detected ions after they have been exchanged on the stationary phase and attached again to detach, so that a detection of these ions is made possible. For this equilibrium reaction following general formula can be given:

Where " A" for the analyte ion, "E" for the Eluention, "S" to stationary phase, "M" for the mobile phase, "X" for the amount of the charge of the analyte ion and "y" for the amount of charge of the Eluentions.

The type of the eluent can be used will depend on the detection mode used. The eluent for conductivity detection can be divided as follows:

• Eluent for conductivity detection with chemical suppression of the fundamental conductivity and
• Eluent for the conductivity detection with electronic compensation of the background conductivity.

Furthermore, the affinities of eluent and solute ( detected ) ions for the stationary phase must be about the same.

The stationary phase and the separation column

As the stationary phase is called the stationary phase, which serves as a carrier material in the separation column. In the high performance liquid chromatography using ion exchange resins, especially the polymer-based, since they are stable on silica in contrast to the column and in the alkaline range and work. Columns, working with support material based on silica, have a slightly higher chromatographic efficiency; However, they work only in the pH range from two to eight. The stationary phase can be divided into the anion exchange not only by the nature of their backbone, but also by the pore size and the capacity of the support material. The ion exchange capacity is defined as the number of ion exchange groups per unit weight of the column filling material. The unit is milliequivalents per gram of resin. The higher the exchange capacity, the longer the retention time of the detected ion. By adding additional eluent, this effect can be partially compensated.

As an example, should be listed here latex anion exchangers, which have the following advantages: The substrate is relatively resistant to mechanical influences, and it guarantees a moderate back pressure. The latex particles have a small size, resulting in a high efficiency of the chromatographic separation column results. Such a separation column is subject to only minor swelling and shrinking processes due to the Pellikularität ( surface functionalization ). It can cheap eluent used ( for example, sodium hydroxide ). The columns can be used for many separation problems apply. The selectivity is affected by the amount of the factors crosslinking degree of the latex polymer and the nature of the functional group in the latex polymer. The ion exchange capacity depends on the following factors:

• Particle size of the substrate ( inversely proportional )
• Size of the latex particle ( proportional)
• Latex coverage on the substrate surface ( proportional).

The suppressor

Suppressors are used when the detection is carried out by means of a conductivity measurement. Your task is to reduce the background conductivity of the eluent, so the suppression takes place before the mobile phase enters the conductivity cell. A suppressor in the simplest case of an ion- exchange column in the hydrogen form, that is, the exchange ions are H or H3O .

First, so-called pillars suppressors were used which were periodic with H to load ( regenerate ). 25 years ago this method was replaced by continuous membrane suppressors. However, also be re- used increasingly in recent columns suppressors, wherein a system of 3 columns is used, which are automatically regenerated and used alternately for suppression.

The following equations serve to illustrate this exchange process:

• The eluent is sodium bicarbonate ( NaHCO3):

• The eluent is sodium hydroxide (NaOH ):

The substances to be detected are transferred in a similar manner to their corresponding acids:

• Analyte is sodium chloride (NaCl):

• Analysis of substance is sodium bromide ( NaBr):

From these equations it follows that only the weakly dissociating carbon dioxide and water are used as a reaction product of Suppressorreaktion the eluent in the conductivity cell. These products are almost non- electrically conductive. The reaction products of the analytes in contrast, are very good conductivity, since they largely dissociate in water:

With the use of carbonate as the basic eluent conductivity may be reduced by removal of the carbon dioxide more. This can, for example, be achieved by a second suppressor or creating a slight negative pressure.

Detector

The detection types are distinguished in electrochemical and spectroscopic methods. Of the electrochemical methods include conductivity detection and amperometric detection. The choice of the type of detection will depend on the separation process and the eluent required for this purpose; the conductivity detection occupies a central role since it can be used universally. The electrical conductivity is expressed in Siemens per centimeter, and has the following formula:

With = length of the conductor, = cross section of the conductor resistance =

The conductivity is proportional to the concentration of ions, to the charge number of ions and the mobility of the ions, so that a quantitative analysis is possible. Furthermore, the conductivity increases with increasing temperature, since the temperature is kept constant, this fact can be neglected. By the detector, the signal is sent to the computer for data processing.

Evaluation with computer

First, a measurement is carried out with a standard solution. In this solution, a known amount of detected ions, which means it is a certain concentration adjusted. Now the sample is introduced into the loop injector and open the loop, then continuously the conductivity readings are transmitted at regular, very small periods of time on the computer. The conductivity, plotted on a graph against time, then results in the so-called " peaks " in the form of a Gaussian curve. Now, the areas of these peaks, caused by the lengthening of the basic conductivity of line in relation to the concentration, which is well known set. When an unknown sample is to be analyzed, one can accurately determine the concentration of the detected ion from the area of ​​the peaks. The surfaces are usually determined from the PC and thus the concentrations. Should be necessary in a solution of an analysis of a plurality of ions, such as, for example, in food analysis, so here the identification of the ions through the characteristic retention times are possible. However, it must for the quantitative determination be guaranteed that do not overlap the peaks of the individual ions, otherwise no clear surfaces can be calculated. This overlap has the choice of a different eluent, or by means of further modifications, such as using a different column can be eliminated.

Example analysis

This measurement was carried out using IonPac CS18 from the column Dionex ( Sunnyvale, USA). The eluent used was methane.

Anions in drinking water (Houston, USA). Addition of chlorite, bromate and chlorate as eluent used was Na2CO3 solution. The measurement was performed with the Metrosep A Supp 7 - conducted 250 from Metrohm.